This invention is in the field of cryostat apparatus for the containment of very low temperature liquefied gasses such as liquid helium and in particular relates to filling and venting structures for such apparatus.
Improvements in cryostat structures for the containment of liquid gas have resulted in progressively lower boil-off rates for the contained cryogen and consequently result in extended containment time for the liquefied gas. Such improvements in cryostat performance result from a variety of improved thermal designs for reduction of thermal losses attributable to conductive and radiative heat transport mechanism. Representative of such advancements are improvements in heat transfer from radiation shields as described in U.S. Pat. No. 4,212,169, and in U.S. Ser. No. 164,451 now U.S. Pat. No. 4,291,541 issued Sept. 29, 1981. Cryostat structures featuring a plurality of nested structures exhibit direct conduction losses through internal bracing required to maintain the spacings of adjacent nested structures. Reduction of such losses is reported in U.S. Ser. No. 164,451 now U.S. Pat. No. 4,291,541 issued Sept. 29, 1981. Radiation losses have been reduced between adjacent surfaces of nested aluminum cryostat structures in accordance with a surface treatment for reduction of emissivity as described in U.S. Ser. No. 879,290.
Where the cryostat further contains a set of superconducting solenoids including a plurality of shim coils, it was recognized that the control channels for such solenoid and shim coils forms a thermal conductive path from ambient surroundings to the central region of the cryostat and that the thermal conductance over such thermal paths could be reduced by selective addressing arrangements for such coils as described in U.S. Pat. No. 4,173,775.
A principal thermal loss path can be identified with convective and radiative losses incurred through the necessary fill and vent tubes for such cryostats. In the prior art, it is known to reduce radiative thermal transport over this path by installation of plane baffles in the fill and vent conduit which baffles occlude a major portion of the cross-section tube. A representative work dealing with the use of plane baffles in this context is reported by Lyman et al. Reduction in cross-section of the fill and vent conduit affects the filling procedure adversely and further, can occasion some concern for safety. A latent hazard may be discerned in the effect of air infiltration down the fill and vent tube. At an appropriate thermal location, liquefaction occurs and the condensed liquid air is mobile along the inner surface of the tube under the influence of gravity. As this condensate creeps down the fill and vent tube toward a cryogen such as liquid helium, solidification occurs and the accumulation of such solid air (hereinafter referred to as "ice") forms a plug in the fill and vent conduit. The occurrence of such a plug can have catastrophic results unless measures are taken to relieve the pressure of the boiling cryogen. It is known in the prior art to provide a relatively large diameter conduit disposed concentrically with respect to the fill and vent tubes and arranged in communication with a "high" pressure relief valve and to establish from the annular region (between such concentric tubes) another pressure relief path through another ("low" pressure) relief valve. The central pressure relief path communicating with the high pressure relief valve is filled with vapor from the boiling cryogen and protected from air infiltration. The annular space communicating with the low pressure relief valve is possibly subject to plugging as described above. In such instance, the alternate path provides pressure relief protecting the prior art cryostat from explosion.